My iPad is activation locked to an Apple ID that no longer exists

So I reset an iPad I owned to try and remove some storage and see if it would run better but my child, not understanding how it worked, activated the activation lock but instead of the previous Apple ID that was being used, it required a deleted apple which had originally been used to set up the iPad which used @ntlworld and I cannot access the iPad anymore

[Re-Titled by Moderator]